<!DOCTYPE html>
<meta charset="utf-8">
<canvas width="960" height="500"></canvas>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>

var canvas = document.querySelector("canvas"),
    context = canvas.getContext("2d"),
    width = canvas.width,
    height = canvas.height,
    radius = 20;

var circles = d3.range(324).map(function(i) {
  return {
    x: (i % 25) * (radius + 1) * 2,
    y: Math.floor(i / 25) * (radius + 1) * 2
  };
});

var simulation = d3.forceSimulation(circles)
    .force("collide", d3.forceCollide(radius + 1).iterations(4))
    .on("tick", drawCircles);

d3.select(canvas)
    .call(d3.drag()
        .container(canvas)
        .subject(dragsubject)
        .on("start", dragstarted)
        .on("drag", dragged)
        .on("end", dragended));

function drawCircles() {
  context.clearRect(0, 0, width, height);
  context.save();
  context.beginPath();
  circles.forEach(drawCircle);
  context.fill();
  context.strokeStyle = "#fff";
  context.stroke();
}

function drawCircle(d) {
  context.moveTo(d.x + radius, d.y);
  context.arc(d.x, d.y, radius, 0, 2 * Math.PI);
}

function dragsubject() {
  return simulation.find(d3.event.x, d3.event.y, radius);
}

function dragstarted() {
  if (!d3.event.active) simulation.alphaTarget(0.3).restart();
  d3.event.subject.fx = d3.event.subject.x;
  d3.event.subject.fy = d3.event.subject.y;
}

function dragged() {
  d3.event.subject.fx = d3.event.x;
  d3.event.subject.fy = d3.event.y;
}

function dragended() {
  if (!d3.event.active) simulation.alphaTarget(0);
  d3.event.subject.fx = null;
  d3.event.subject.fy = null;
}

</script>
